How can I get rid of air buildup in a siphon system?

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In summary, the siphon system is not working well because small amounts of air bubbles are building up and eventually the siphon is lost. The problem is greatest where the siphon has the greatest possible height and so pressure reduction. There are many solutions available, but the most likely one is a battery operated vacuum pump that turns on when a certain amount of gas is gathered at the top of the siphon.
  • #1
grizzster
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I have been trying to control the water level in a water hole that I have using a siphon system. I have a 1.5 inch hose that is submersed in the upper water hole and the hose travels slightly up hill for a 100 feet then goes slightly down hill for the other 100 feet into a large barrel of water which just overflows during the siphon process. The water level of the top of the barrel is the level I want to achieve in the upper water hole. Everything works very well for about 6-7 hours of siphon but eventually small amounts of air bubbles build up into large air pockets and siphon is eventually lost. I can't really change where the hose is. How can I release this eventual build up of air automatically without losing the siphon. Is there some kind of valve? Or some trick that can be used to stop the build up of air from traveling down the hose? I have been reading that there are various reasons why the gas build up is happening. I can't really change that but I am hoping of a way to dispose of the gas? Any help would be greatly appreciated. Thanks
 
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  • #2
Welcome to PF.
This is a problem with ground water that contains dissolved gasses. The problem is greatest where the siphon has the greatest possible height and so pressure reduction. It is not really the distance the siphon travels diagonally but the height it lifts water that determines the depression at the top of the siphon.
There are many solutions available.

You could attach a large reservoir to the top of the siphon. Low pressure gas would gradually collect there until you turn off the normally open bottom valve and refill the reservoir with water through the normally closed top valve. Reservoir size would need to be large for a siphon with a big height difference. That gas might be worth collecting if it is something like methane.

A small positive displacement vacuum pump could operate to very slowly draw low pressure gas from the top of the siphon. It could be hand, solar or wind powered.

By using a bundle of fine tubes rather than one big pipe, gas bubbles coming out of solution towards the top of the siphon would remain entrained in the liquid flow until they redissolved or bubbled out at the end of the siphon.

Keep the siphon tube cool and out of the sun. Gasses are more soluble in water at low temperatures than high. The water from underground will probably be cool.

If the gas is mostly CO2 then you might consider a chemical absorber like used in a scuba re-breather. That could be expensive. http://en.wikipedia.org/wiki/Soda_lime
 
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  • #3
Thanks so much for the reply Baluncore. I will try to use one or more of these suggestions to rid of this problem.
 
  • #4
grizzster said:
Any help would be greatly appreciated. Thanks
Thanks so much for the reply Baluncore.
Yes... very good advice from Baluncore.

Also, if you can, check for any pin holes in the hose...

I have been reading that there are various reasons why the gas build up is happening.
You have probably read this ... but maybe other people haven't.

According to the Talk page, the article could be a bit better... siphons due seem to be somewhat complicated.

Look at the View history page too... I think I made the last edit... :approve: ... Lol.
 
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  • #5
we have decided to install a small battery operated vacuum pump that only turns on when a certain amount of gas is gathered at the top of the siphon. Will let you know how it works
 
  • #6
So I am still fighting with gasses collecting in this siphon system. We added a container at the highest point of the siphon system with a shut off valve. The container was filled with water. Our idea was to for the container to catch all the gases and replacing the water in the container with the gas. The shutoff valve would allow us to remove the container to remove gas and refill with water. If this would have worked we would have used a larger container so that it wouldn't have to be refilled as often.
We primed the siphon and once it was working we opened the valve. To our surprise the water was sucked out of the container so fast that it flattened the container. Fail! Our thought now is to get a container that won't collapse. Any other suggestions? Is there some kind of auto air bleeder or degassing valve that would work in a siphon system? It certainly would make this a lot more simple to achieve. We can get the siphon to work for a few hours and then the gasses build up enough that it losses the siphon. Getting rid of these gassed is proving to be a lot more difficult that I had thought.
 
  • #7
You will need a transparent vertical canister between two ball valves, all mounted on a T connector at the highest point on the siphon tube.

Repeat the following cycle whenever you need to remove low pressure gas from the top of the syphon tube.
1. Close the bottom valve.
2. Open the top valve, (expect it to suck in some air).
3. Fill canister with water through the top valve to displace gas.
4. Close the top valve.
5. Open the bottom valve so gas can again rise into the canister as it appears in the siphon.

The bigger the canister, the less often you need to flood it again.
The greater the siphon head the greater will be the partial vacuum, so a greater volume canister will be needed to remove the same mass of gas.
 
  • #8
grizzster said:
So I am still fighting with gasses collecting in this siphon system. We added a container at the highest point of the siphon system with a shut off valve. The container was filled with water. Our idea was to for the container to catch all the gases and replacing the water in the container with the gas. The shutoff valve would allow us to remove the container to remove gas and refill with water. If this would have worked we would have used a larger container so that it wouldn't have to be refilled as often.
We primed the siphon and once it was working we opened the valve. To our surprise the water was sucked out of the container so fast that it flattened the container. Fail! Our thought now is to get a container that won't collapse. Any other suggestions? Is there some kind of auto air bleeder or degassing valve that would work in a siphon system? It certainly would make this a lot more simple to achieve. We can get the siphon to work for a few hours and then the gasses build up enough that it losses the siphon. Getting rid of these gassed is proving to be a lot more difficult that I had thought.
A bleeder valve won't work because as you found, the system is negatively pressurized at the top. You need a rigid container, capable of handling a pretty healthy vacuum without collapsing. Your (Baluncore's) solution was a good one, you just need a stronger container.
 
  • #9
Baluncore's suggestion to use a bundle of small tubes, is very clever indeed. It would make a fun project for students to find out how small the tubes have to be.
 
  • #10
this is what we tried so far. Seems to work so we need to get a larger canister now. I would like to try a collection of smaller tubes though. I just have so much money invested in this right now.
20150719_094353_resized.jpg
20150719_094459_resized_1.jpg
 
  • #11
Foot valve on water source side pressure release valve on top of the collector, 100ft header pump on barrel side, with timer to force out the air.
 
  • #12
A couple of quick questions. 1) what sort of acceptable range in the receiving vessel. 2) What sort of initial and overall topographical head are you dealing with? It seams as if a conventional dosing siphon would maintain your levels. This type siphon resets itself with each dose and as a byproduct it clears the dissolved gasses when the transport purges.
 
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  • #13
Ketch22 said:
A couple of quick questions. 1)It seams as if a conventional dosing siphon would maintain your levels. This type siphon resets itself with each dose and as a byproduct it clears the dissolved gasses when the transport purges.

Wow, I learn something every day on PF. I never heard of a dosing siphon, so I looked it up. I found this very interesting page. Very clever.

http://www.siphons.com/how-siphons-work.html
 
  • #14
There are a few simple considerations if a Dosing siphon is to be used. First is the Topographical head from dose tank to receiving tank. If it is consistently negative then it is quite easy. If it has a positive component along the run it can be done but with special consideration. They will not develop enough pressure to overcome any positive topographical head.
A second consideration is that the void volume of the initial transport pipe needs to be sufficient to dissipate the air bubble. If pipe size due to outside issues is going to be restrictive there would need to be further consideration.
Last is the dose volume. A dosing siphon is easy to calculate in that it triggers at a predictable height above the bell and it will brake at a predictable height below the bell. This distance (for instance in inches) multiplied by the Gallons per inch of the tank you are using will tell you the overall dose volume. One can by varying relationships change the dose volume as required. The only restriction being the initial surge going into the primary pipe.

Advantages for you is: 1)That they operate strictly by gravity and without external applied power. 2)They are self priming at each dose. 3) As the transport drains out each cycle and the new cycle flushes the pipe, dissolved or suspended gasses do not affect them by virtue of not accumulating.
If this of interest to your application we can continue towards a solution. If not you still learned a cool new tool.
 
  • #15
I'm still having issues with this siphon system. I would be interested in trying a Dosing siphon but I don't know much about them and not sure if it would push the water uphill for a hundred feet. On another note, if I used a 1 1/2" initial source hose up to the crest and then used a 2" hose downhill to the lower barrel would this created a faster flow on 1 1/2" hose thus making the dissolved gases flow out instead of accumulating at the crest?
 
  • #16
It still is a question of overall head. The length of pipe makes a bit of difference that we can address in a few. From the high point in your feed tank to the high point of the transport line the height of the lift determines the depression in pressure. This would be what is causing the water to flow up hill and generally what could be considered to be causing the dissolved gasses to come out of solution. (1) we would need to know what that lift height is not the lateral run of the pipe.
Now speaking to the length of transport and your suggestion for a pipe change. What you are suggesting would have a negative affect on the function. The pictures you shared show what looks like a PVC spa hose. There are several variations that are loosely the same as schedule 40 rigid. The outside diameter is the same so glued fittings still work. Without knowing the exact hose I need to shift to generalities.
For all plumbing systems the positive or negative pressure is generated by the topographical head not the physical size of the transport. A water column that is 33.6 feet in height in a 1/2 " pipe will generate exactly 1 atmosphere of pressure. A water column of the same height in a 2" pipe will generate exactly the same pressure.
There is to be considered that a larger pipe has more square inches for the pressure to act against. However this also affects the volume. Your 1 1/2" hose will hold between 11.5 and 12.9 gallons per 100' of pipe. Thus anything entering (including gasses) the system must travel for whatever amount of time it takes for the requisite volume to transit. When you step up to 2" hose the volume is 17.9 to 20.3 gallons per 100'. With that increase in area the flow rate (which is all drawing through the same orifice) is slower. An interruption in flow such as a bubble will flow quickly up the hill but slowly down which is where it will create the greatest disruption.
Interestingly enough the siphons, even the ones that were accidental in nature, that are the most efficient (and thus the hardest to prevent) are the ones that are in a smaller tubes. With the flow speed up they are constantly flushing themselves of violations. Also on a uniform hill ( say 10% up gradient and 10% down gradient) the uniform piping allows for a net 0 when disrupted.
If you do decide to go with a dosing siphon you will find that it is critical that the initial piping, which is right after the siphon be sufficiently large to allow the entire air bubble to evacuate allowing manifold fill for the rest of the system.
 
  • #17
There are a couple of things that I would check.
Firstly; if the well recharge rate is low, then the water level in the well will fall as water is removed. At some point air will enter the siphon tube, how is that prevented. What regulates the flow rate from the well so as to keep sufficient water in the well.
Secondly; the gas that accumulates in the high reservoir is at a low absolute pressure. It is therefore very difficult to pump it out efficiently.It would be better to keep it as small bubbles in a fast flow. That suggests a smaller pipe would be better.
 
  • #18
The recharge rate right now is quite fast..it is a ditch that surrounds a couple of acres and water runs in at one end. We had a large hole dug in the ditch at the siphon intake with a large garbage can which has the end of the hose in it to keep debris out and let's water in through holes. The hose travels approximately 100 feet to the crest then gradually downward another 100 feet to a barrel full of water. The top of the barrel is about two feet lower than the intake hose. If the water in the ditch drops two feet the water level in the ditch and the lower barrel will be at the same level and siphon should stop. If water then increases in the ditch the siphon should just keep going since both ends of the hose is still submerged. This was our hope anyway. We never get that far along because I assume the dissolved gases is breaking the siphon eventually( in less than 12 hours). Water runs out of the barrel quite fast in the beginning. It is surprising to me that it ever stops. If we just left the hose on the ground instead of the barrel in would eventually drain the ditch if not for dissolved gasses but we don't want it drained otherwise siphon will need to be reprimed after a rain. I live 2 hours away. Sounds like our only solution is a few smaller hoses. Would raising the crest higher make a faster flow? What size small hose would be ideal for keeping air bubbles flowing?
 
  • #19
grizzster said:
Would raising the crest higher make a faster flow?
Raising the crest would be a disaster. You are already too close to the height limit which is why the water is close to, or is “boiling” at environment temperature and releasing all the dissolved gasses.
The height limit of a siphon will depend on the minimum atmospheric pressure when there is a low pressure system and the higher density of water at low tube temperatures. It is lower still for salty water which has a higher density.

grizzster said:
What size small hose would be ideal for keeping air bubbles flowing?
One or several smaller tubes will work only while the water keeps moving, but thin tubes have much greater flow resistance so they will each carry much less water. Keep the thick riser tube, but replace only the downhill section with a 6mm = ¼ inch nylon pneumatic line which will be low cost and has cheap fittings. Experiment; when you find a size that pulls the gas through the system, work out the flow you need and duplicate the downhill run with several parallel lines.

Relying on the height to stop the siphon as inlet falls is risky as it means the pressure difference is close to zero and so the fluid flow is very slow. That slow flow will not remove gas bubbles through small tubes.
 
  • #20
You still seem to be missing the point. It is the relative height of the inlet and the outlet that generates the siphon. If there is a crest in the center that is of negligible effect other than being a cause for priming the siphon. I am assuming that the crest is not 100 feet above the inlet tank. 100 feet of lateral distance is of little consequence.
As to the size of hose that would be best. How much water do you need to transfer and at what rate? Also we need to know the relative height difference between inlet and outlet. This determines the pressure that is available for use. The hose would be the orifice and then the flow rate can be calculated with the data using the orifice flow formula or more accurate a calculation involving a Reynolds number if available for the hose that is in use.
 
  • #21
Just to walk through the stuff a bit. Let's do a quick little napkin scratch engineering with some stuff we know.

A typical 5/8" garden hose flows between 3 and 5 gallons per minute when the household pressure is between 30 psi and 60 psi.
These pressures roughly equate to 1 bar to 2 bar which would be 33 feet of head to 66 feet of head.
Assuming that your water level at the source tank is maintained 30 feet of elevation above the discharge point this would maintain 1 bar.
Garden hose is not a good smooth tube but it is functional. The pressure loss due to 250 feet of hose would cause an additional 50% or more in flow.
Best estimate for actual flow rate given 250 feet of 5/8" garden hose with a 30 head would be somewhere around 1 gallon per minute.
There are 1440 minutes in a 24 our day, thus a garden hose should transfer around 1400 gallons per day if flow is maintained.

1 inch pipe holds very close to 7 gallons per 100 feet of pipe.
Volumetrically 5/8" hose is around 40% of that size varying widely with actual hose type.
The hose should hold approximately 2.8 gallons per 100 feet or 35 feet per gallon.
This makes the velocity of the water in the hose close to 35 feet per minute which is inside the realm of possibility and keeps a good recharge rate to assist with gasses.

One side advantage to garden hose is that the siphon can pre primed with one of those economical rotary pumps that uses a cordless drill and connects to the garden hose.

Hopefully this helps in your calculation process and gives you a little guidance.
 
  • #22
As the height of a siphon increases, the amount of gas coming out of solution increases. A siphon can not be expected to lift the theoretical limit of 9.8 metre = 32 ft. As the height approaches that limit all the gas comes out of solution, just before the cold water boils and blocks the pipe at the crest with cold steam.

There would be no problem with big gas bubbles forming and blocking the siphon if the dissolved gas that came out of solution remained in the fluid as separate small bubbles. The water is not pure and deionised, so we really cannot expect that in a rural setting.

It is counter-intuitive, but having gas dissolved in siphon water is not always a disadvantage. It is interesting to note that, if the many small bubbles did not coalesce, the water would appear cloudy and the density would decrease. Then the siphon might significantly exceed the 32 ft limit for water, something that can only happen if gas was first dissolved in the water.

The best double advantage that I can see for tall siphons is a surfactant or foaming agent that might be added to the inlet water to reduce bubble coalescence and so reduce the fluid density. Consider what would happen if you placed a bar of soap in the ditch near the siphon inlet, or mixed in a “blood and bone” fertiliser powder.
 
  • #23
Ketch22 said:
Just to walk through the stuff a bit. Let's do a quick little napkin scratch engineering with some stuff we know.
These pressures roughly equate to 1 bar to 2 bar which would be 33 feet of head to 66 feet of head.
.
Ugh just noticed a significant mistype. 30 to 60 pounds would be roughly 2 to 4 bar. which is 60 to 120 feet of head.
An assumed head of 30 feet would still be 1 bar which makes the flow rate approximately 1.5 gpm. with consideration for friction loss it should still be at about 1 gpm.
 
  • #24
Ketch22 said:
... An assumed head of 30 feet would still be 1 bar which makes the flow rate approximately 1.5 gpm. with consideration for friction loss it should still be at about 1 gpm.
Is the 30 ft head not balanced by the water rising in the inlet pipe, so there is only 32 ft - 30 ft = 2 ft of head remaining to move water through the entire pipe system.
 
  • #25
Baluncore said:
Is the 30 ft head not balanced by the water rising in the inlet pipe, so there is only 32 ft - 30 ft = 2 ft of head remaining to move water through the entire pipe system.
This would be essentially true. It is my understanding that that is not the situation in this case. From the water level in the inlet tank any increase in altitude must be counted as a negative. The total head from the high point would be added to that pressure to calculate the actual head. Alternatively one can look at only the reduction from the inlet tank water level to the outlet of the hose. This is the actual head however it is very important to remember as you said that a rise from inlet to high point creates a low pressure area which does not react well for keeping the gasses in solution.

I was assuming that the lower end of the hose was 30' ( for sake of example) below the inlet water level. if this had a 2-3 foot rise along the way it would create minimal change to the napkin scratching I penned out earlier.
 
  • #26
Ketch22 said:
I was assuming that the lower end of the hose was 30' ( for sake of example) below the inlet water level.
grizzster said:
The top of the barrel is about two feet lower than the intake hose. If the water in the ditch drops two feet the water level in the ditch and the lower barrel will be at the same level and siphon should stop. If water then increases in the ditch the siphon should just keep going since both ends of the hose is still submerged.
So it appears there is almost no head to overcome resistance to flow through the siphon pipe.
 
  • #27
So far the assumption seems to be that it's dissolved air in the water. Probably is but have you eliminated the possibility of a small air leak into the hose? It would have to be quite small as the syphon does work for awhile.
 
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  • #28
grizzster said:
The top of the barrel is about two feet lower than the intake hose.

That's not right. The syphon will keep going until the water level drops below the height of the intake hose and then air will enter the intake.

syphon.jpg
 
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  • #29
On the original question - you could use a self acting pump to remove the trapped vapour/air .

Two possibilities :

(a) A venturi pump .

(b) A hydraulic motor driving a mechanical pump .

Edit : In fact a venturi pump fitted directly in the down flow pipe and a suction pipe going back to the highest point may be all that is needed .
 
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  • #30
Is this actually what you have?

syphon.jpg

The syphon should run until the level in the ditch on the left falls to the control level (top of barrel on right).
 
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  • #31
CWatters said:
...have you eliminated the possibility of a small air leak into the hose?
He should have... ?
OCR said:
Also, if you can, check for any pin holes in the hose...
 
  • #32
4yrs later what happened? Did we figure it out? A level hose without a smooth interior surface will trap air and over hours collect and stop flow. A smooth schedule 40 with 0 breaks fittings joints will solve it. Air is the worst enemy of water flow. Air is a Phantom plug that stumps most.
 
  • #33
Nidum said:
On the original question - you could use a self acting pump to remove the trapped vapour/air .

Two possibilities :

(a) A venturi pump .

(b) A hydraulic motor driving a mechanical pump .

Edit : In fact a venturi pump fitted directly in the down flow pipe and a suction pipe going back to the highest point may be all that is needed .

Reviving this dead thread to ask about what you proposed here. I'm looking at a similar situation to the original post where I'll be using creek water with plenty of dissolved gasses, and the system will be located in a remote difficult to access area, so I want to make sure any accumulated gasses at the apex of the siphon are constantly removed so I don't need to regularly visit the site.

You suggested the use of a venturi at a point on the downward flowing pipe with the idea that it can pull the air from the apex of the siphon. My understanding of a venturi is that the pipe constriction causes a sharp increase in velocity through the constriction with an associated pressure drop. That pressure drop across the constriction is what would pull the air down through the suction pipe from the siphon's apex. But wouldn't the vacuum generated by the venturi have to be greater than the static vacuum that would exist at the top of the siphon in order to pull the air? Is this even possible in a system like a siphon? Since the action of the venturi is doing work in order to pull the air, what would the effective net head loss be across the entire siphon? That is, how much higher would the siphon intake need to be than the siphon outlet in order to have the net head needed in order for the venturi to do that work?

This all assumes that the flow through the siphon does not exceed the recharge rate of the source. If it did, then all if this is almost useless conjecture anyway, since the flow through the siphon would eventually slow to a stop as the source water was drawn down, preventing the venturi from being able to pull the air from the apex. There's still the potential for the gas to accumulate at the apex while the flow is low and the venturi is inoperable, eventually breaking the siphon. There might need to be the addition of a float and valve on the intake side that closes the intake when the source water level drops to a point where the net head difference across the siphon starts getting too small to effectively operate the venturi.
 
  • #34
@KDunc Welcome to PF.
You seem to understand the requirements.
1. A valve that will close the input before air can enter the siphon tube.
2. A U-bend or water trap at the outlet that will prevent air running back up the siphon tube.
3. Some way to prime the siphon tube.
4. Some way of removing the gasses from the near vacuum at the top of the siphon tube.

A venturi is not a solution to 3 or 4.
The solution will be situation and detail dependent.
 
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  • #35
Baluncore said:
@KDunc Welcome to PF.
You seem to understand the requirements.
1. A valve that will close the input before air can enter the siphon tube.
2. A U-bend or water trap at the outlet that will prevent air running back up the siphon tube.
3. Some way to prime the siphon tube.
4. Some way of removing the gasses from the near vacuum at the top of the siphon tube.

A venturi is not a solution to 3 or 4.
The solution will be situation and detail dependent.
Thanks for the reply. I thought as much - a venturi seems like it wouldn't work, and certainly wouldn't be able to compensate for the dynamic nature of such a system. Proven methods like a vacuum pump and/or some way to regularly re-prime the system are likely the best way to handle this situation.

To continue to explore out-of-the-box methods: a common method for priming a siphon is to close the inlet and outlet valves, then open a valve at the apex of the siphon and completely fill the siphon with water from this apex point. What if we left this apex valve open and connected it via a small diameter pipe to the bottom of an open-top tank that had a continuous flow of water coming into it? The siphon effect would also aggressively pull water down from the tank, but the small diameter of the intermediate pipe would restrict this flow of water. So long as the apex tank was recharged faster than the outflow into the siphon, then would this be an effective method to prevent the accumulation of gas at the apex of the siphon? The issue I see is that the intermediate tank-to-apex pipe would need to be small in order to decrease the tank recharge needs, but the low diameter of this pipe may also prevent the gas from flowing through it in the reverse direction, preventing its escape from the siphon. This idea would only be useful in the event that a small stream is available to supplement the siphoning of a much larger creek.
 

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